I. Technical Field
The present invention relates to a valve gear and, in particular, to a valve gear capable of preventing the sticking between the valve stem of a steam valve and a valve bush supporting the valve stem.
II. Description of the Related Art
A main steam valve in a steam turbine is used under harsh conditions of high temperature and high pressure, and further takes the task of controlling high-speed steam flow. Under high temperature, the surface of a metal is in an activated state to react with the high-temperature steam present in the atmosphere to produce an oxide film. This oxide film has different adhesion strength to the base metal, depending on the composition of the base metal and the atmospheric conditions, and undergoes delamination at every repeated opening-closing movement of the valve. The delaminated matter is deposited locally in the recessed portions of the valve stem by the sliding of the valve stem so as to fill in the clearance between the valve bush and the valve stem. Consequently, the sticking of the valve stem is caused as the case may be. Accordingly, at the time of the periodic inspection of a steam turbine, it is necessary to conduct maintenance work for removing the oxide film by disassembling the members surrounding the valve stem. Additionally, the generation amount of the deposition substance is anticipated, and accordingly the clearance between the valve stem and the valve bush is set at a sufficiently large value. Therefore, the steam amount leaking from the portion surrounding the valve stem is increased to cause problems such as the decrease of the thermal efficiency of a whole plant.
A proposal for solving the problem of such the sticking is disclosed in Patent Document 1 Japanese Patent Publication No. 1-28269. Japanese Patent Publication No. 1-28269 proposes that the main body of a valve stem and a bush (hereinafter referred to as a valve bush) be formed from materials having almost the same linear expansion coefficient, that a Ni alloy be building-up welded on the outer surface of a sliding part of the valve stem, and that the building-up welded Ni alloy portion be subjected to a nitriding treatment.
According to the proposal of Japanese Patent Publication No. 1-28269, because a Ni alloy is provided on the outer surface of a sliding part of the valve stem, little oxide film is generated and hence the occurrence of sticking can be prevented. Furthermore, because the main body of the valve stem and the valve bush are formed from materials having almost the same linear expansion coefficient, it is possible to ensure the clearance between sliding parts formed between the two even when the valve is used at high temperatures.
Although Japanese Patent Publication No. 1-28269 describes that austenite stainless steels, 12% Cr-based stainless steels and low-alloy steels (Cr—Mo(—V) steels) are used in the main body of the valve stem and the valve bush, there is no specific description related to Ni alloys.